From DE 10 2012 020 908 A1 a method for operating a drive-train of a motor vehicle is known. The drive-train comprises a drive machine, a manual transmission and a drive axle which can be brought into driving connection with a further axle by a pressure-controlled switching device. The switching to produce a driving connection between the further axle and the drive axle takes place in a driving-situation-dependent manner.
For motor vehicle transmissions, which by actuating the switching device transmit to the connected axle a braking force applied to the drive axle by a brake mechanism, an ambivalent situation arises in which, in order to protect the drive-train, in particular an axe gearbox of the connectable axle, only a specified torque should be transmitted thereto so as to avoid prolonged loading with too high a torque. This conflicts with the legal requirement that in particular with agricultural or municipal motor vehicles the full braking power must be transmitted to the connected axle in order to achieve the required retardation in the event of maximum load and when towing an additional, unbraked trailer. The torque transmitted during this can exceed the specified torque that can be transmitted for long periods to the connected axle without damage, so by virtue of a monitoring logic system the driving connection can be interrupted by the switching device in order to safeguard the axle transmission of the connected axle against overload. This, however, would result in a braking force loss if the motor vehicle could mainly transmit braking forces only by way of the connected axle, particularly due to a dynamic axle load distribution.